Turbochargers deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine can replace a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle.
Turbochargers are a type of forced induction system which uses the exhaust flow entering the turbine housing from the engine exhaust manifold to drive a turbine wheel (51) which is located in a turbine housing (2). The turbine wheel is solidly affixed to a shaft to become the shaft and wheel assembly. The primary function of the turbine wheel is extracting rotational power from the exhaust gas and using this power to drive the compressor.
The compressor stage consists of a wheel (20) and it's housing (10). The compressor wheel (20) is mounted to a stub shaft end of the shaft and wheel assembly and is held in position by the clamp load from a compressor nut. Filtered air is drawn axially into the inlet (14) of the compressor cover by the rotation of the compressor wheel at very high RPM. The turbine stage drives the compressor wheel to produce a combination of static pressure with some residual kinetic energy and heat. The pressurized gas exits the compressor cover through a compressor discharge (15) and is delivered, usually via an intercooler, to the engine intake.
Compressor surge occurs when the compressor attempts to deliver more massflow to the engine than is possible at the existing engine operating condition, i.e., aerodynamic stall. The compressor stage begins to oscillate in terms of pressure: mass flow, speed, and net aerodynamic thrust. This oscillating instability can be quite damaging to the turbocharger, also producing an irritating noise which can be heard by the driver and is usually described as a “bark” or “squawk”. The location of surge for a given compressor design can be described as a function of pressure and mass flow at a given rotational speed.
Compression ignition (CI) engines have air induced directly into the cylinder. The air is compressed by the piston on the compression stroke, and the fuel is injected into the heated compressed air just before the piston reaches top dead center (TDC).
In turbocharged CI engines the mass flow of air is delivered by the turbocharger output, and the fuel flow is metered and injected directly into the combustion chamber. Some CI engines are equipped with throttles.
Spark Ignited (SI) engines may mix the combustion air with fuel in the inlet manifold. The resultant air-fuel mixture is controlled by a throttle valve prior to entering the combustion chamber. The throttle valve, or plate, is typically located in a throttle body with relatively close tolerances and has the ability to close off the airflow to the engine. SI engines may also inject fuel directly into the cylinder.
“Tip-in” is the term used to refer to the action of driver's foot depressing the accelerator pedal to adjust engine load. Engine speed may remain the same, driving up a hill, for example, or the engine may increase from low engine speed to higher engine speed. “Tip-out” is the term used to refer to the opposite action of the driver's foot lifting off the pedal.
Compressor Recirculation Valves (CRV) and the ducts connecting the exhaust from the compressor to the inlet to the valve and the exhaust from the valve to the inlet of the compressor, collectively hereafter the “CRV system”, are used today in many SI and CI engines, or engines employing throttle plates for air control, typically to prevent surge. The closing of the throttle plate, at accelerator pedal tip-out, for example, closes the duct from the compressor discharge to the engine inlet and causing a sudden reduction in compressor flow resulting in the compressor stage to going into surge. CRV systems in general deliver air from the compressor discharge duct (the duct connecting the compressor discharge to the engine inlet or intercooler, depending upon the engine configuration) to the ducting upstream of the compressor inlet, or directly to the compressor inlet.
The output of the compressor recirculation valve may be connected to the compressor inlet by ducting to the compressor inlet or ducting to the compressor wheel. Alternatively, the compressor recirculation valve and ducting may be part of the compressor cover casting. Similarly the input to the compressor recirculation valve may be a duct connecting the CRV to the compressor discharge, or the input to the CRV may be part of the compressor cover.
The CRV system may be connected to the compressor by ducting to the compressor in several ways. Some systems have piping from the compressor discharge to the CRV valve, and then from the CRV valve to the ambient inlet duct from the air cleaner or even the compressor inlet duct; some have parts of this arrangement only (for example the CRV may be mounted to the compressor cover discharge with a flexible pipe to the compressor inlet); some have the CRV integrated directly into the compressor cover. The invention teaches the method of directing the recirculated air to the compressor wheel, regardless of the design by which the CRV assembly may be mounted.
When the CRV valve is opened, a volume of high pressure compressor discharge air radially enters the compressor inlet, joining the axially flowing main stream, causing a sudden inrush of air through the aperture in the compressor cover inducer (defined below) and can cause cavity resonance noise akin to that of blowing over the opening in the neck of a bottle. This noise may also be irritating to the driver. In FIG. 2A the inlet air (61) is sucked axially into and through the compressor wheel (20) compressed and ejected from the compressor wheel through the diffuser (11) into the volute (12), where the velocity component of air from the compressor wheel and the diffuser is collected and translated to pressure. Typically there exists a CRV (80) which controls a recirculation flow of pressurized air (62) from the compressor discharge (15). With the CRV (80) in the open position, or modulated towards the open position, the recirculation air (64) is admitted to a recirculation duct (16) and thence admitted into the compressor inlet (e.g., the region directly upstream of the compressor wheel leading edge (24).
Because the duct (16), when fluidly connecting the compressor discharge (15) to the CRV; and the CRV to the compressor stage inlet (14); dumps the unidirectionally, generally radially flowing, high pressure recirculation air directly into the axially flowing main inlet airflow (61) and then to the inducer area (14a) the velocity distribution across the plane of leading edges (24) of the compressor wheel is not uniform. This lack of circumferentially uniform airflow velocity is sucked into the compressor wheel which results in some flow aligned with the blades and some flow not aligned with the blades. Some blades are fully loaded while some blades may be in a negative pressure condition.
FIG. 2B is a view of the section “A-A” of FIG. 2A. In FIG. 2B, the generally radial airflow is depicted in plan view. In this section view, the airflow (64) from the discharge duct (15, 15a) is introduced into the CRV duct (16) connecting the CRV to the compressor inlet. The generally radial airflow (65) from the CRV duct dumps directly into the axial main compressor inlet flow (61) without the opportunity for the merged airflow to be conditioned; thus, the air ingested by the compressor wheel is generally unconditioned from a velocity standpoint.
This duct configuration causes a pressure gradient across the inlet to the compressor wheel which, when severe, can cause an uneven blade loading of the compressor wheel blades. This excitation can result in high cycle fatigue (HCF) of said blades. In addition this duct configuration also causes the surge noise described above. This phenomenon also causes a reduction in stage efficiency when the CRV is open.
So it can be seen that “dumping” compressor recirculation flow directly into the compressor wheel causes not only noise, but also the potential for compressor wheel blade failure and a general loss of efficiency when the CRV is open.